This application claims the priority of 100 24 570.6, filed May 19, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system and a method for operating a fuel cell system.
U.S. Pat. No. 3,972,731 discloses a fuel cell system in which a compressor which is connected to a turbine via a common shaft is used to supply air to the fuel cell cathode. The fuel cell anode is exposed to a hydrogen-rich reformate generated by an endothermic reaction in a steam reformer that is thermally coupled to a catalytic burner. The hot and moist cathode exhaust gas is cooled substantially in a water separator, dehumidified, and then heated again by a catalytic burner arranged in the exhaust system. The heated exhaust gas drives the turbine which is coupled to the compressor.
To supply air to the cathode side, fuel cell systems usually require a compressor. In methanol-operated fuel cell systems, high system pressure is inherently advantageous, for example, to improve the water balance of the system. Furthermore, the efficiency of the fuel cell rises as a result of the higher oxygen partial pressure, and pressure losses in the fuel cell system are minimized.
However, an elevated pressure level is associated with a higher uptake by the compressor, which leads to efficiency losses and, for the same useful power, an increase in the size of the fuel cell system.
One object of the invention is to provide a PEM fuel cell system, and a method for operating such a system, which makes beneficial use of cathode exhaust gas to increase the useful power of the system.
This and other objects and advantages are achieved by the fuel cell system according to the invention, in which the fuel cell unit is preferably operated with pure hydrogen gas. A catalytic burner is arranged in the cathode exhaust-gas flow path, with the anode exhaust-gas flow path opening into the catalytic burner and/or into the cathode exhaust-gas flow path upstream of the catalytic burner. The combined, catalytically converted fuel cell exhaust-gas flow can be introduced at elevated temperature into an expansion machine.
A fuel cell system of this type, in which the fuel cell unit is operated with hydrogen that is as pure as possible, has the advantage that it is possible to work with a low hydrogen excess in the part-load range and increasing the hydrogen excess in the high-load range. In this manner a large quantity of exhaust-gas energy is available.
In a particularly preferred embodiment of the fuel cell unit according to the invention, fuel cell stacks are connected together both in series and in parallel in terms of flow, and the hydrogen excess can be optimized over the part-load range and the full-load range. If the fuel cell unit is operated with pure or highly purified hydrogen, it is ensured that even the last cell or the last fuel cell stack, as seen in the direction of flow, receives hydrogen of sufficient purity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.